The present invention relates to a turbomolecular pump having a magnetic bearing system to be used for pumping to high vacuum and especially relates to a turbomolecular pump adaptable for extremely high vacuum pumping by promoting an efficiency of heat transfer due to heat radiation from a rotor of the pump.
A conventional turbomolecular pump has a casing provided with an inlet port at one end portion and an outlet port at the other end thereof and also has a rotor supported by magnetic bearings floatingly in the casing. The rotor has rotor vanes arranged axially therealong in multi-steps. Furthermore a stator is provided in the casing, and the stator has stator vanes inserted between the rotor vanes with very small gaps therebetween in such a way that the stator vanes are fixed by spacer rings on an inner wall of the casing. A motor is provided coaxially with the rotor for driving the rotor rotationally in the casing and a number of revolution of the motor is measured by a sensor.
When using such a conventional turbomolecular pump having a magnetic bearing system of the structure stated above, the rotor rotates in a condition of completely floating in the vacuum environment during the outgassing operation. Namely, the rotor is floating in a status of non-contact with the casing and the stator in the vacuum.
There are, however, some problems with the above conventional turbomolecular pump having a magnetic bearing system.
Since the rotor is floating in a condition of completely non-contact with a casing or a stator, an efficiency of heat transfer between the rotor and the neighboring elements is low. Namely, there is no heat transfer by heat conduction, because the rotor does not contact the casing and the stator, and there is no heat transfer by heat convection, because the rotor is in an ultra vacuum environment and in non-contact status with the casing and the stator. Consequently, heat transfer is performed only by heat radiation. The rotor, in general, is made of aluminum alloy, and the stator vanes and the spacer rings mounted for fixing the stator vanes are also made of aluminum alloy. This aluminum alloy has low emissivity, that is 0.04 approximately, and stainless steel which is used frequently as a structural material for vacuum apparatus has about 0.4 of emissivity. Accordingly, even by radiation, a high efficiency of heat transfer would not be much expected.
Further, when carrying out a baking treatment by using a heater outside of the casing for removing gases adsorped to the surface of the rotor, the temperature of the rotor cannot reach a sufficient baking temperature because of less heat exchange between the rotor and the casing or the stator.
Moreover, during operation of the rotor, there is a phenomenon of heat generation due to eddy current loss in the motor, and also another phenomenon of heat generation due to eddy current loss in the magnetic circuit dynamically controlling the magnetic bearings which support the rotor.
On the other hand, the rotor during operation exists in an idealistic adiabatic condition of no heat conduction and no heat convection as well as very small heat radiation due to the aluminum alloy having a very small emissivity, and also the rotor exists in a non-contact condition at the bearing portions. Therefore, the temperature of the rotor is raised to very high degree by heat generation at the rotor where the operation temperature should be kept 50.degree..about.70.degree. C., so that the extremely high vacuum can not be realized because of outgassing from the rotor.